In the production of oil, gas and other valuable minerals from subterranean wells, large numbers of pipe and tubular sections are often employed, such sections typically connected by threaded couplings. Interior surfaces of these tubular sections and their associated couplings are frequently subjected to temperatures in excess of 350.degree. F. pressures as high as 20,000 PSI, and environments which may be highly corrosive, such as those produced by the presence of hydrocarbons, CO.sub.2 and H.sub.2 S in the presence of water. The use of secondary and tertiary enhanced recovery methods in oil fields may tend to further aggravate the situation.
Pipe sections used in oil fields usually have a tapered, exteriorly threaded male end called a pin member. Such pin members are threaded into couplings, collars or integral female pipe sections, their threaded ends being referred to as box members. These box members have an interiorly threaded tapered end which corresponds with their respective pin members. As can readily be appreciated, these components, when produced from steel are subject to attack by corrosion.
Corrosion in metals is caused by the flow of electricity from one metal to another metal or from one part of the surface of one piece of metal to another part of the same metal where conditions permit the flow of electricity. Further, a moist conductor or electrolyte must be present for this flow of energy to take place. Energy passes from a negative region to a positive region via the electrolyte media. Several types of corrosion mechanisms exist, including: bi-metallic corrosion, erosion-corrosion (also known as impingement), stress corrosion, intergranular corrosion, and galvanic corrosion.
Electrical contact or coupling of dissimilar metals frequently causes increased corrosion, this form of corrosion generally referred to as galvanic corrosion. Galvanic corrosion is quite prevalent and troublesome, occurring in a wide variety of circumstances. For example, coupling aluminum and iron pipe together will result in very rapid corrosion of the aluminum pipe section. The galvanic corrosion mechanism may be illustrated by considering the effect of electrically connecting zinc to platinum immersed in sea water. Under these conditions, the platinum is inert and does not corrode, while the zinc is attacked. The reactions occurring on the surface of the zinc are the anodic oxidation of zinc to zinc ions, and the cathodic reduction of dissolved oxygen to hydroxide ions. If the electrical potentials of these two metals are measured, the platinum would be found to have a positive potential, while the zinc would be found to have a negative potential. As may be appreciated, as the potential difference increases, galvanic corrosion increases.
Obviously, from a corrosion standpoint, the replacement of steel tubulars and associated hardware with materials less subject to corrosion would be highly desirable in gas and oil field applications, if it were practical. While the use of corrosion resistant alloys for corrosion control have demonstrated superior corrosion resistance properties, they are quite costly and exhibit complex manufacturing and handling constraints. Non-metallic components, such as fiberglass casing, tubing, sucker rods and the like are finding their way into oil field applications. Performance limitations, including service loads, pressures and temperatures, restrict the across-the-board replacement of metallic hardware, however.
In practice, to guard against galvanic corrosion, insulative coatings are frequently applied. In order for a coating to be used on tubular sections and threaded couplings to protect the metal substrate from corrosion, the coating must be resistant to attack and maintain its adherence to the metal substrate under the harsh conditions referred to above. In various oil and gas applications, steel pipe is provided with a lining of corrosion-resistant material. For example, it is known to bond to the interior of the pipe various epoxy-based coatings, as well as coatings containing polyethylene, polyvinyl chloride and other thermoplastic and thermosetting materials.
Of the various polymeric coating materials, arylene sulfide polymers have gained wide acceptance and are well known in the art. (See U.S. Pat. No. 3,354,129 to Edmonds, Jr. et al.) Generally, these polymers consist of a recurring aromatic structure coupled in repeating units through a sulfur atom. Commercially available arylene sulfide polymers which have been used for coating oil and gas pipes and pipe couplings are polyphenylene sulfides. The polyphenylene sulfides used in oil and gas applications exhibit high melting points, outstanding chemical resistance, thermal stability and are non-flammable. They are also characterized by high stiffness and good retention of mechanical properties at elevated temperatures as well as the ability to deform smoothly, thereby, for example, preventing the galling of threads, even at high thicknesses. U.S. Pat. No. 3,744,530, issued to Perry, describes polyphenylene sulfide coated pipes, wherein the polyphenylene sulfide coating also contains a filler, such as iron oxide, in an amount of between 5% to 30%.
While polymeric coated pipes and couplings have gained wide acceptance in applications requiring corrosion protection, the cracking of such coatings during installation and in use tends to limit the insulative effect of such coatings, increasing the likelihood that galvanic corrosion will take place. This is particularly true in the pin-end area where cracking occurs during assembly, when the threaded portion undergoes deformation. Moreover, the polymeric coatings of threaded couplings are particularly prone to cracking due to the stresses imparted during assembly.
Despite the advances in the corrosion protection of oil-field tubulars, couplings and associated hardware, a need exists for hardware of improved corrosion resistance which possess the mechanical properties necessary to serve in oil field and similar service.